This invention relates to aircraft navigation in general and more particularly to a digital Tacan processor for use in aircraft.
Tacan equipment is used on most military aircraft to provide an indication of the distance, and bearing to any selected Tacan station. In a Tacan system, a ground station transmits Tacan pulses at a rate of approximately 2700 per second. The ground station antenna rotates 15 times per second and has an antenna directivity pattern which has a cardioid component to it. This causes the pulses received by the aircraft to have a 15 hertz modulation of plus or minus 20% amplitude and a phase which depends upon the aircraft's bearing with respect to the Tacan station. The transmitter also sends a reference pulse code once each antenna cycle at a fixed point in the antenna rotation to establish an absolute reference phase. The aircraft equipment determines the bearing by comparing the phase of the amplitude modulated signal with the reference phase pulses. For further accuracy, a ninth harmonic is also superimposed on the antenna directivity pattern producing a 135 hertz amplitude modulation. This will be present at the receiver at phase coherence with the 15 hertz modulation described above and permits making a finer determination of bearing in a manner similar to that used in two speed servo systems.
To measure range the aircraft transmits an inquiry to the ground station and determines the range by using the time interval of a reply to its inquiry. Many extranious transmissions such as replies to other aircraft and the providing of other functions are also made by the ground station (approximately 2700/sec). The range pulses are the same pulses used for determining bearing. In other words only one set of pulses is transmitted by the ground station with the bearing information being imposed thereon through amplitude modulation and the range information by the time interval between a reply and its inquiry. The total pulses which the aircraft will receive will include all replies, i.e., replies from its own interrogations and replies to interrogations from other aircraft. In addition, within the total number of pulses, will be the coded reference pulses and additional pulses which are randomly added by the ground station to achieve the desired number of pulses necessary for the system to operate properly i.e. approximately 2700/sec. Out of all these received pulses, the aircraft must be capable of identifying the reply to its own interrogation by corrolation over a number of inquiries. Generally, during the initial acquisition mode, a high inquiry rate of approximately 150 inquiries per second is used to speed up acquisition. Thereafter, a tracking mode is entered with a reduced inquiry rate which is less than thirty per second. During the acquisition mode, a conventional Tacan distance measuring equipment (DME) system employs slow scan with scans from zero towards a maximum range and takes typically 10 seconds.
Most prior art systems for processing Tacan systems have been of an analog nature and have been quite costly, due to the complexity of the equipment. This is particularly true of the equipment necessary to determine bearing. Because of this high cost, most commercial airlines have elected to use the less complex VOR equipment for bearing along with a Tacan DME.
Thus, it can be seen that there is a need for less expensive, accurate and faster operating Tacan equipment which can be universally used in both military and commercial aircraft.